Black component generation

ABSTRACT

A system and method for generating achromatic components for output incorporate receiving input color data defined in a first color space. The received input color data is converted to intermediate color data defined in an intermediate color space. Typically the intermediate color space is defined only in terms of chromatic components. A black or achromatic component is then calculated. The black or achromatic color component is a function of both the input color data in the first color space and the intermediate color data in the intermediate color space. This black or achromatic color component is associated with the intermediate color data for further processing and output.

BACKGROUND OF THE INVENTION

The present invention relates to the digital color image processingarts. It finds particular application in conjunction with generating ablack component for output in CMYK color space, and will be describedwith particular reference thereto. However, it is to be appreciated thatthe invention is applicable to various transforms between input andoutput color spaces.

Color in printed digital images results from the combination of alimited set of colors over a small area in densities selected tointegrate the desired color response. This is accomplished in manyprinting devices by reproducing so called “separations” of the image,where each separation provides varying values of a single primary color.When the separations are combined together, the result is a full colorimage.

The particular color of each separation depends on the “color space”being implemented. Examples of color space models include, RGB, CMY,CMYK, Lab, Yes, YIQ, HSV, HLS.

In practice, color images are commonly printed in acyan-magenta-yellow-black (CMYK) color-space. This color space is basedupon the CMY color-space, but attempts to improve the quality of “black”in the image and reduce use of color inks, or toners, or dies, or thelike. In theory, images can be printed using the CMY color space, with amixture of the three colors producing black. In practice, however,printing with only cyan, magenta, and yellow inks often does not producethe highest quality black, but instead results in a muddy brownishoutput due to impurities in the inks, the particular paper or otherimage recording media used, and the partial reflection of light insteadof its complete absorption into the inks. Furthermore, select use ofblack ink in place of the primary colors reduces expense and minimizesthe total amount of ink used which is often desirable in ink-jet andother printing applications where the ability of the recording substrateto absorb ink is limited.

Methods for converting to the CMYK color space include those referred toas “under color removal” (UCR) and “gray component replacement” (GCR).UCR/GCR methods vary, but commonly involve examining the individualpixels of an image using the lowest or “darkest” of the threecyan-magenta-yellow colors to determine an amount of black to be added(Gray Component Replacement). One or more of the CMY colors are thenadjusted to account for the addition of black ink (Under Color Removal).For example, if a given pixel of an image is represented in the CMYcolor space by C=0.5, M=0.4, and Y=0.25, then the black or K value wouldbe based upon the lowest or Y value. In a 50% gray component replacement(GCR) method, K=(50% of Y)=0.125. In a typical under color removal (UCR)step, the remaining CMY values would then each be reduced by 0.125 sothat the resulting UCR/GCR pixel is represented by C=0.375, M=0.275,Y=0.125, and K=0.125. Of course, other UCR/GCR methods are known, buteach seeks to determine the level of black for a given pixel, based onlyon the intermediate color space, e.g. CMY.

The present invention contemplates a new method and apparatus togenerate output color components that yield increased output quality.

SUMMARY OF THE INVENTION

A method of generating a black component of an output color includesreceiving input color data defined in a first color space and convertingthe received input data to intermediate color data defined in a secondor intermediate color space. A first black component factor iscalculated from the input color data and a second black component factoris calculated from the intermediate color data. The first and secondblack component factors are then combined.

In accordance with another aspect of the present invention, thereceiving input color data includes receiving color separation valuessuch as RGM, CMY, LAB, YES, YIQ, HSV, HLS, and the like.

In accordance with another aspect of the present invention, theconverting step includes converting the received input color to CMY.

In accordance with another aspect of the present invention, thecalculating a first black component factor includes applying a functionto the input color separations dependent upon predefined thresholdvalues for all or some of the input color separations.

In accordance with another aspect of the present invention, thecalculating a first black component includes applying a function to HSVinput color data.

In accordance with another aspect of the present invention, thecalculating a second black component factor includes calculating thesecond black component factor from the converted CMY values.

In accordance with another embodiment of the present invention, a methodof outputting digital color image data in an image output terminalincludes converting input color components corresponding to a color in afirst color space to intermediate components in a second color spaceincluding only chromatic components. An output black component is thenderived from both the intermediate components and the input components.

In accordance with another aspect of the present invention, the methodfurther includes adjusting the intermediate components based on thederived output black component. This adjustment includes gray componentreplacement and under color removal and the like.

In accordance with another aspect of the present invention, the methodfurther includes outputting data based on both the intermediatecomponents and the output black component.

In accordance with another aspect of the present invention, the derivingan output black component includes applying a defined process to theinput components, applying a defined process to the intermediate colorcomponents and combining the processed input components with differentlyprocessed intermediate components.

In accordance with another aspect of the present invention, thedifferently processed intermediate components include minimized CMYvalues.

In accordance with another embodiment of the present invention, a systemfor creating an achromatic component for an output color space frominput color space including only chromatic components includes areceiver which receives input color separations in the input color spaceand a converter that converts input color separations to intermediatecolor separations. The system further includes an achromatic componentgenerator in data communication with the receiver and the converter. Theachromatic component generator calculates an achromatic color separationfrom functions including at least one input color separation andintermediate color separation.

In accordance with another aspect of the present invention, the inputspace is defined in HSV and the functions include a first functiong(H,S,V), and a second function h depending on the intermediate colorseparation. The system further includes a calculator that calculatesg(H,S,V) by implementing the equation.

${g\left( {H,S,V} \right)} = {1 - \left\{ \begin{matrix}{{a_{H}S^{2}} + {b_{H}S} + c_{H}} & {{{for}\mspace{14mu} S} > {S_{t}\mspace{14mu}{and}\mspace{14mu} V} > V_{t}} \\{1 - \left( {1 - \frac{V}{V_{t}}} \right)^{2}} & {{{for}\mspace{14mu} S} \leq {S_{t}\mspace{14mu}{and}\mspace{14mu} V} < V_{t}} \\{\left( {{a_{H}S^{2}} + {b_{H}S} + c_{H}} \right)\left\lbrack {1 - \left( {1 - \frac{V}{V_{t}}} \right)^{2}} \right\rbrack} & {{{for}\mspace{14mu} S} > {S_{t}\mspace{14mu}{and}\mspace{14mu} V} < V_{t}} \\1 & {{{{for}\mspace{14mu} S} \leq \;{S_{t}\mspace{14mu}{and}\mspace{14mu} V} > V_{t}}\mspace{11mu}}\end{matrix} \right.}$

In accordance with another aspect of the present invention, theconverter converts the input color separations to CMY.

In accordance with another aspect of the present invention, the systemfurther includes a post processor that adjusts the intermediate colorseparations based on the achromatic color separation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating preferred embodiments, and are notto be construed as limiting the invention.

FIG. 1 shows a system illustrated as a functional block diagram whichsuitably practices the present invention;

FIG. 2 is a depiction of color space showing variables and thresholds atselected positions within that color space and their effect on blackcomponent generation;

FIG. 3 is an illustration of black generated as a function of V and S;

FIG. 4 is an illustration of black generated as a function of V, S, andhue angle;

FIG. 5 is a graph of possible weighting values for modulating H; and

FIG. 6 is a graph of the function with weights for modulating H.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference now to FIG. 1, a system for creating a black orachromatic component from a color represented by an input color spaceincludes a receiver 10 which receives input color separations in theinput color space. The discussion herein will focus on input color spaceRGB, but those skilled in the art will appreciate that alternate inputcomponents can be depicted in a variety of different formats such asLab, LCH, HSV and the like. Typically, input color space is defined inthree dimensions and the components describing individual or particularcolors are collectively called tristimulus values while individualcomponents are referred to as color separations. The input colorseparations 12 are then supplied to an intermediate color generator 14or means for converting the input color separations 12 to intermediatecolor separations 16. Typically, the intermediate color separations 16are generated through lookup tables, one-minus algorithms or otherfunctions. As used herein intermediate color space will be referred toas CMY although those skilled in the art will appreciate that otherintermediate color spaces may also be used.

In data communication with receiver 10 and intermediate color generator14 is an achromatic component generator 20. The achromatic componentgenerator 20 receives a representation of the color in both the inputcolor space and the intermediate color space. As more fully discussedbelow, the achromatic component generator 20 applies a function f toboth representations to calculate a black or achromatic component 28. Inone preferred embodiment, the achromatic component generator 20 appliesdifferent processes or functions to the input components 12 and theintermediate components 16. From this a first black component 22 and asecond black component 24 are derived, and combined into achromaticcomponent 28. The achromatic component 28 is forwarded to post processor30 which combines the achromatic component 28 with intermediatecomponents 16 and then processes the color separations as is known tothose skilled in the art, for example gray component replacement, inklimit, tone reproduction curves and the like. The system then outputscolor separations via an image output terminal 34 which can include aprinter, copy machine, xerographic device, and the like.

A new method for black component generation takes into account not onlythe intermediate color values 16 (for example CMY) but also thecorresponding input color values 12 (for example RGB or HSV).Specifically a black or achromatic component 28 is generated accordingto the following strategy (FIG. 2):K=f(input color, intermediate color)  EQ(1)

The function f is customized according to the specific requirements. Forexample, for an inkjet printer, it is preferable to start turning onblack later because the black dots increase the graininess of skintones, or sweeps of yellow to black for light colors it is preferable touse three color black or process black. A specific embodiment ofequation (1) will be described below.

The black component 28 is generated as a function of the first blackcomponent 22 e.g. the HSV values of the input RGB color separations 12and the second black component 24, e.g. min(C,M,Y) 16, (where C=255-R,M=255-G, Y=255-B are the intermediate colors) as follows:K=f(H,S,V,C,M,Y)=g(H,S,V)*h(C,M,Y)  EQ(2)where the first black component 22 is represented asg(H,S,V)=1−g ₁(V)*g ₂(S,H)  EQ(3)and the second black component 24 is represented ash(C,M,Y)=min(C,M,Y)  EQ(4)g₁ and g₂ are defined in equation (5) below.

$\begin{matrix}{{g_{1}(V)} = \left\{ {\begin{matrix}{1 - \left( {1 - \frac{V}{V_{t}}} \right)^{2}} & {{{if}\mspace{14mu} V} < V_{t}} \\1 & {otherwise}\end{matrix}{and}} \right.} & {{EQ}\mspace{14mu}\text{(5a)}} \\{{g_{2}\left( {S,H} \right)} = \left\{ \begin{matrix}{{a_{H}S^{2}} + {b_{H}S} + c_{H}} & {{{if}\mspace{14mu} S} > S_{t}} \\1 & {otherwise}\end{matrix} \right.} & {{EQ}\mspace{14mu}\text{(5b)}}\end{matrix}$where V_(t) and S_(t) are experimentally determined thresholds. ForV>V_(t) and S<=S_(t) no black is generated. V_(t)=0.86 and S_(t)=0.52are example values that worked well for an ink-jet system. Thesethreshold values usually gave better results when V_(t)>S_(t).

The amount of black produced is given by the function f and depends onthe position of the input color component 12 e.g. HSV space, and on theintermediate color component 16 e.g. CMY values. This dependence on theinput color space HSV is depicted in FIG. 2. Those skilled in the artwill recognize that the thresholds and parameters may likely bedifferent for different print modes and media.

Power 2 in the definition of g₁ proved to be less aggressive as a linearfunction in certain parts of the color space. For dark and lowsaturation colors a dependence of the amount of black generated only onV is sufficient.

For g₂, a curve similar to g₁ was first generated depending only onsaturation S and the threshold S_(t):g ₂(S)=1−((S−S _(t))/(1−S _(t)))² if S>S _(t) and 1 otherwise  EQ(6)

But high saturation light colors had high graininess (such as almostpure yellow in black to yellow sweeps) due to the presence of the bigvolume black drops. Accordingly, in another embodiment, the amount ofblack generated depends on the hue angle for select high value highsaturation input colors, which improves the image quality noticeably.

FIG. 3 is a plot of the product g₁g₂ scaled by a factor of 100 as afunction of V and S when g₂ depends only on saturation (see Equation 6above) and g₁ is defined by Equation 5a.

Again, the 3^(rd) axis is the value of the g₁g₂ product. A value of 0(for example at point C where S=V=1)) means 100% black generation and avalue of 1 (100—for example at point A where S=S_(t) and V=1) means noblack generated. As inferred above, this led to the presence of blackdots for very light (V close to 1), high saturation (S close to 1)colors. To avoid these artifacts for S=V=1 (at point C) the value of theproduct g₁g₂ is shifted higher than 0, such that less black is produced.

Additionally the position of C was determined to vary with the hue angleH. In short generating less black as value and saturation increase andresults in a curve of the type shown in FIG. 4, such that the positionof point C varies as a function m of H (m(H)). To achieve this, a seconddegree curve was fitted (for V=1) to the following data/aim points:A(S_(t),1)C(1, m(H))B(0.5(S_(t)+1),0.75)g₂ was derived by solving system (7) below.

The first point (A) shows that no black should be generated for S≦S_(t).

The second point (C) introduces the dependence on H as a function m(H),where H is the hue angle. If m(H)=0, then the dependence described inFIG. 3 and Equations 5a and 6 is obtained.

The third point (B) is an anchor point movable for different cases andalso provides enough data to solve the system of equations obtained byrequiring that the function defined in Equation 5b passes through thepoints A, B and C.

The system is:

$\begin{matrix}\left\{ \begin{matrix}{{{a_{H}S_{t}^{2}} + {b_{H}S_{t}} + c_{H}} = 1} \\{{a_{H} + b_{H} + c_{H}} = {m(H)}} \\{{{a_{H}\left( {S_{t} + 1} \right)}^{2} + {2{b_{H}\left( {S_{t} + 1} \right)}} + {4c_{H}}} = 3}\end{matrix} \right. & {{EQ}\mspace{14mu}(7)}\end{matrix}$

The solution of system (7) is:

$\begin{matrix}\left\{ \begin{matrix}{a_{H} = \frac{{2{m(H)}} - 1}{\left( {1 - S_{t}} \right)^{2}}} \\{b_{H} = \frac{{2S_{t}} - {m(H)} - {3{m(H)}S_{t}}}{\left( {1 - S_{t}} \right)^{2}}} \\{c_{H} = \frac{1 - {2S_{t}} + {{m(H)}S_{t}} + {{m(H)}S_{t}^{2}}}{\left( {1 - S_{t}} \right)^{2}}}\end{matrix} \right. & {{EQ}\mspace{14mu}(8)}\end{matrix}$

Once more if m(H)=0 then g₂ from Equation 5b is the same as g₂ fromEquation 6.

The function m is a modulated value of H, because the same strategy forblack generation was desired for certain groups of colors. This will beexemplified below after the definition of ‘m’ is introduced. ‘m’ isdefined as in Equation 9:m(H)=W ₁ cos(3H)+W ₂  EQ(9)where W₁ and W₂ are selected such that 0≦m(H)≦1 which means 0≦W₁, W₂≦1,W₁+W₂≦1, W₂−W₁≧0, which limits the possible pairs of (W₁, W₂) values tothe shaded region 50 in the plane W₁W₂ from FIG. 5.

The graph of m is in FIG. 6, for W₁=0.1 and W₂=0.9. These values werechosen after extensive testing of various (W₁, W₂) pairs.

Black is generated in the same way for R, G, and B (in which casem(H)=W₁+W₂=1 for W₁=0.1 and W₂=0.9) and in a different way for C, M, andY (in which case m(H)=W₁−W₂=0.8 for W₁=0.1 and W₂=0.9) as shown in FIG.6.

Finally for high saturation, low value colors (as compared to thresholdsV_(t) and S_(t)) the black generation method is further refinedincluding a dependence on all three attributes of the input colorseparations, for example: H, S, and V.

Overall, the black generation strategy is defined as a continuousfunction of H, S, and V, as in Equation 10.

$\begin{matrix}{{g\left( {H,S,V} \right)} = {1 - \left\{ \begin{matrix}{{a_{H}S^{2}} + {b_{H}S} + c_{H}} & {{{for}\mspace{14mu} S} > {S_{t}\mspace{14mu}{and}\mspace{14mu} V} > V_{t}} \\{1 - \left( {1 - \frac{V}{V_{t}}} \right)^{2}} & {{{for}\mspace{14mu} S} \leq {S_{t}\mspace{14mu}{and}\mspace{14mu} V} < V_{t}} \\{\left( {{a_{H}S^{2}} + {b_{H}S} + c_{H}} \right)\left\lbrack {1 - \left( {1 - \frac{V}{V_{t}}} \right)^{2}} \right\rbrack} & {{{for}\mspace{14mu} S} > {S_{t}\mspace{14mu}{and}\mspace{14mu} V} < V_{t}} \\1 & {{{{for}\mspace{14mu} S} \leq \;{S_{t}\mspace{14mu}{and}\mspace{14mu} V} > V_{t}}\mspace{11mu}}\end{matrix} \right.}} & {{EQ}\mspace{14mu}(10)}\end{matrix}$

Those skilled in the art will appreciate that further improvements maybe made by making the low saturation, low value curve also huedependent.

The invention has been described with reference to the preferredembodiments. Modifications and alterations will occur to others upon areading and understanding of the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. In an image output terminal, a method of outputting a digital colorimage comprising: converting input components corresponding to a colorin a first color space to intermediate components in a second colorspace, where the second color space includes at least one intermediatecomponent that is different from the input components; and, deriving anoutput black component from both the intermediate components and theinput components, wherein said deriving includes: applying a definedprocess to the input components; applying a different process to theintermediate components; and, combining the processed input componentsand the differently processed intermediate components; wherein the firstcolor space includes HSV and the applying a defined process comprisesapplying a function including H, S and V, where the function isdifferent depending on the position of a V value with respect to athreshold.
 2. In an image output terminal, a method of outputting adigital color image comprising: converting input componentscorresponding to a color in a first color space to intermediatecomponents in a second color space, where the second color spaceincludes at least one intermediate component that is different from theinput components; and, deriving an output black component from both theintermediate components and the input components, wherein said derivingincludes: applying a defined process to the input components; applying adifferent process to the intermediate components; and, combining theprocessed input components and the differently processed intermediatecomponents; wherein the first color space includes HSV and the applyinga defined process comprises applying a function including H, S and V,where the function is different depending on the position of an S valuewith respect to a threshold.
 3. The method of outputting a digital colorimage as set forth in claim 2, where the applying a different process tothe intermediate components comprises minimizing CMY values.
 4. In animage output terminal, a method of outputting a digital color imagecomprising: converting input components corresponding to a color in afirst color space to intermediate components in a second color space,where the second color space includes at least one intermediatecomponent that is different from the input components; and deriving anoutput black component from both the intermediate components and theinput components; wherein the first color space comprises HSV and thederiving comprises: applying a function to the intermediate components;and for input components including V below a V threshold and S below anS threshold, applying a function varying with V and the V threshold. 5.The method of outputting a digital color image as set forth in claim 4,further comprising outputting data comprising the intermediatecomponents and the output black component.
 6. The system as set forth inclaim 4, where the input color space is defined in HSV and the functionsinclude a first function g(H,S,V), and a second function h depending onthe intermediate color separation, the system further comprising acalculator which calculates g(H,S,V) by implementing the equation:${g\left( {H,S,V} \right)} = {1 - \left\{ \begin{matrix}{{a_{H}S^{2}} + {b_{H}S} + c_{H}} & {{{for}\mspace{14mu} S} > {S_{t}\mspace{14mu}{and}\mspace{14mu} V} > V_{t}} \\{1 - \left( {1 - \frac{V}{V_{t}}} \right)^{2}} & {{{for}\mspace{14mu} S} \leq {S_{t}\mspace{14mu}{and}\mspace{14mu} V} < V_{t}} \\{\left( {{a_{H}S^{2}} + {b_{H}S} + c_{H}} \right)\left\lbrack {1 - \left( {1 - \frac{V}{V_{t}}} \right)^{2}} \right\rbrack} & {{{for}\mspace{14mu} S} > {S_{t}\mspace{14mu}{and}\mspace{14mu} V} < V_{t}} \\1 & {{{{for}\mspace{14mu} S} \leq \;{S_{t}\mspace{14mu}{and}\mspace{14mu} V} > V_{t}}\mspace{11mu}}\end{matrix} \right.}$ where coefficients a_(H), b_(H) and c_(H) aredifferent functions of hue angle, and S_(t) and V_(t) are thresholdvalues for saturation and value, respectively.
 7. The system as setforth in claim 4, where the intermediate color separations are definedin CMY.
 8. The system as set forth in claim 4, further comprising apost-processor that adjusts the intermediate color separations based onthe calculated achromatic color separation.